1. Field of the Art
The present invention relates to an isolated DNA sequence coding for a polypeptide which has enzymatic activity of glycerol 3-phosphate acyltransferase (referred to hereinafter as ATase) as is produced by Arabidopsis thaliana Heynhold.
2. Related Art
Lipid components of the plant biomembranes change from a liquid-crystalline phase to a gel phase when the environmental temperature decreases, which is accompanied by changes in the properties of these biomembranes. It is presumed that the biomembranes cannot fulfill its intact functions in the gel phase as they lose the selective permeability, which results in disfunctions of cells. Among the lipids, phosphatidylglycerol (referred to hereinafter as PG) changes from the liquid-crystalline phase to the gel phase at a high temperature and thus tends to gelate at a high temperature. Therefore, the temperature sensitivity of biomembranes greatly varies with the properties of PG. In this connection, the tendency of gelation of PG molecules depends on the composition of fatty acids that constitutes the PG. The transfer of an acyl-group to glycerol 3-phosphate (referred to hereinafter as G-3-P) is carried out by the ATase. That is, the transfer reaction of an acyl-group from an acyl-carrier protein (referred to hereinafter as ACP) to G-3-P is catalyzed by the ATase.
In plants, fatty acids are synthesized only in chloroplasts. Acyl-ACPs, the substrates of the ATase, are mainly palmitoyl-ACP (referred to hereinafter as 16:0-ACP) and oleoyl-ACP (referred to hereinafter as 18:1-ACP). The selection between these two substrates by the ATase depends on a property of the ATase itself, that is, substrate selectivity. The substrate selectivity of the ATase has been investigated for a variety of plants. For example, the ATase from chilling-resistant plants, spinach and pea, has a high substrate selectivity to the 18:1-ACP [Eur. J. Biochem., 129 (1983), 629-636], and the PG of these plants remains in liquid-crystalline phase even at a relatively low temperature. On the other hand, since the ATase from chilling-sensitive plants does not distinguish 18:1-ACP from 16:0-ACP and transfers both fatty acids in an almost equal proportion, the PG of squash gelates at a relatively high temperature (details will be described later).
Among the ATases of plants, that of squash, a chilling-sensitive plant, is the only one for which the complete amino acid sequence has been elucidated. The sequence has been applied for a patent as "a DNA sequence coding for glycerol 3-phosphate acyltranspherase" by the present inventor (Japanese Patent Laid-Open Publication No. 235594/1989). The complete amino acid sequence of an ATase from chilling-resistant plants is elucidated for the first time by the present invention in which the sequence is derived from the enzyme of Arabidopsis thaliana Heynhold. While the amino acid sequence of the ATase of Arabidopsis thaliana Heynhold is as a whole similar to that of squash, it has regions which are greatly different from that of squash. The difference in the substrate selectivity of the ATases is probably accounted for by this difference in the amino acid sequences.
It is indicated that a "transit peptide" is present at the N-terminal domain of proteins which are synthesized in cytoplasm and transferred to chloroplasts. It has been demonstrated that such a transit peptide is necessary for their transfer [Nature, 313 (1985), 358-363].